1. Field of the Invention
The present invention relates to an ink jet recording apparatus for discharging ink droplets onto the recording medium to obtain a desired ink image.
2. Related Background Art
Conventionally, recording apparatuses which have features of printer, copying machine and facsimile, or are employed as the output device for composite electronic equipment or workstation containing a computer, a work processor and so on, are configured to record the image (including characters or symbols) onto the recording sheet (recording medium) such as a paper or a plastic thin plate, based on image information. Such recording apparatuses can be classified into an ink jet system, a wire dot system, a thermal system, and a laser beam system, depending on the recording method.
In a serial type recording apparatus of the serial scan system of scanning in a direction crosswise to the conveying direction of recording sheet (sub-scan direction), the recording can be performed on the whole recording sheet by repeatedly performing the operation of recording (scanning) the image by recording means mounted on a carriage moving along the recording sheet, after recording one line, feeding (pitch conveying) the sheet by a predetermined amount, and then recording (scanning) the image at the next line on the recording sheet which is stopped again.
On the other hand, in a line type recording apparatus which performs the recording only by the sub-scanning (conveying) of the recording sheet in a conveying direction, the recording is performed on the whole recording sheet by repeatedly performing the operation of, after setting the recording sheet at a predetermined recording position, recording one line collectively feeding (pitch conveying) the sheet by a predetermined amount, and further recording the next line collectively.
Among the above recording apparatuses, a recording apparatus of the ink jet system (ink jet recording apparatus) performs the recording by discharging the ink from recording means (recording head) onto the recording sheet. The ink jet recording apparatus has the advantages in which recording means can be made compact, the higher definition image can be recorded at higher speed, the ordinary paper is usable for recording without needs of any special treatment, the running cost is lower, there is less noise owing to the non-impact method, and the color image is readily recorded by using multi-color inks.
In the ink jet recording apparatus, the recording medium supplied by a sheet feeder has its conveyance surface held by a planar platen in the image recording area, and after the image is formed by the recording head, it is exhausted by paper exhausting means provided downstream in the conveyance direction.
In the conventional ink jet recording apparatuses, there was typically provided a certain gap between the recording head and the recording medium. Therefore, there were some problems as follows:
(1) If there is too wide gap between the recording head and the recording medium, the impinging accuracy of ink droplets may degrade, resulting in poor recording quality.
(2) If there is too narrow gap between the recording head and the recording medium, the gap between the recording head and the recording medium is removed due to deformation of the recording medium, causing sliding with each other, which causes the output image to be not only contaminated but also the recording head or recording apparatus main unit to be out of order due to nozzle clogging with the paper powder or dust which has entered the nozzle.
(3) Further, with the spread of color printers in recent years, there are increasingly many instances for recording the image having a great amount of ink impingement. In these instances, a wavelike undulation (thereinafter referred to as waving) may arise on the recorded face because the recording medium has swollen with the ink sinking into the recording medium. In such instances, the distance for ink droplets to reach the recording medium may vary, thereby causing the impinging position to be offset, resulting in uneven recording and poor image quality.
(4) Also, owing to the increased recording speed and the multicolor recording, the recording head is provided with a longer nozzle array, requiring an elongated interval of carrying the recording medium between the rollers disposed before and after the recording head in the conveyance direction of the recording medium, which further fosters the problem (3).
(5) With such longer nozzle array of the recording head, it is necessary that if the trailing end of the recording medium gets rid of conveying means in forming the image up to the trailing end of the recording medium, the recording sheet is conveyed with a conveying force of paper exhausting means to accomplish the recording. At this time, if the trailing end of recording medium is deformed, or floated, the recording sheet may, in some cases, contact with or slide on the recording head.
As shown in FIGS. 5 and 6, in the conventional constitution having a platen and exhausting means, if the recording medium waved, that waving portion would significantly float directly up the platen surface, so that the distance L0 between the set record surface and the ink jet recording head would greatly vary.
In this case, the variations in the impingement accuracy of discharged ink may not only occur due to partial differences in the distance between the recording head and the recording medium, but also the ink discharge timing in each nozzle is not normally identical, so that the varying ink droplet flight distance causes the impinging position to be offset, making high quality image recording even impossible.
Further, if the waving is large such as L1&gt;L0, the recording head and the record face of recording medium may slide, as above described, thereby contaminating the output image, and causing damage to the recording head or recording apparatus main unit. To avoid the above drawback, the L0 value is taken to be as large as possible to relieve the abuses of waving, and eradicate the sliding between the recording head and the recording medium, but the recording accuracy and quality may significantly degrade, and it is possibly undesirable to obtain excellent results of image recording.
Also, even in the cases where there is no waving, particularly the end portion of recording medium may be raised up because the recording medium itself is stored in unfavorable condition. In recording the image on such recording medium, it often became difficult to record on the end portion of recording medium owing to the above reason, and was obliged to take a narrower image recording area reluctantly.
Further, to suppress the waving of the recording medium due to ink droplets as above described, there has been conventionally known an ink jet recording apparatus (Japanese Laid-open Patent Application No. 61-280965) for placing the recording medium into contact with the platen by generating a negative pressure, and an ink jet recording apparatus (Japanese Laid-open Patent Application No. 3-27949) for placing the recording medium into contact with the conveying belt by generating an electrostatic force. However, either of these recording apparatuses requires a device for generating negative pressure or static electricity, giving rise to complex constitution, larger size, and increased manufacturing costs of the recording apparatus.